Dynamic cervical plate

ABSTRACT

A dynamic subsidence plate is described having a first plate member and a second plate member in sliding engagement that may infinitely subside between a first and second assembled position. The plate includes a lock assembly associated with the first and second plate members. The lock assembly includes a ramp portion, an interference portion, and a bearing member situated between the ramp portion and the interference portion. The lock assembly of the plate is configured to allow movement of the first member with respect to the second member in a first direction in an infinite number of positions between a first assembled position and a second assembled position. Further, the lock assembly alternatively prevents movement of the first member with respect to the second member in an opposite second direction between the first and second assembled positions.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/757,226, filed Feb. 1, 2013, which application is adivisional of U.S. patent application Ser. No. 11/900,914, filed Sep.13, 2007, the disclosures of which are incorporated herein by reference.

FIELD OF THE TECHNOLOGY

The present invention relates to a bone plate, and more particularly itrelates to such a dynamic bone plate for use in spine.

BACKGROUND OF THE INVENTION

Spinal fixation plates can be used for a variety of conditions,including for example, providing added strength and rigidity afterfusion of adjacent vertebral bodies for securing vertebrae togetherwhere an intervening vertebral body has been removed and replaced.Generally, a spinal fixation plate is applied to the anterior side ofaffected vertebrae to span at least one affected disc space. Forexample, a spinal fixation plate may be applied to adjacent vertebralbodies where at least a portion of a disc has been removed and a spinalfusion spacer has been inserted.

Generally, a spinal plate may be attached to the anterior of two or morevertebral bodies for the purpose of immobilizing, stabilizing, and/oraligning those vertebrae. Additionally, such a plate may be used, forexample, to supplement the function of an intervertebral spacer orartificial disc, to prevent an intervertebral spacer from being expelledfrom an intervertebral disc space and/or to act as a support forbiocompatible bone graft material that is implanted in the disc space.

Orthopedic fixation devices such as spinal plates may be coupled to bonewith fasteners inserted through openings in the plates. The fastenersmay or may not be secured to the plate. It is known to secure suchfasteners to a bone plate, for example, through the use of threads onthe fastener and matching threads on the plate, though other means ofsecurement are available. Such a screw-plate interface may decrease theincidence of loosening of the fixation assembly post-operatively. It isalso known that a bushing may preferably be disposed in each plate holeto receive the fastener to permit polyaxial movement so that thefastener may be angulated at a surgeon-selected angle. While polyaxialmovement of fasteners through set plate hole locations may increaseattachment alternatives of the fasteners themselves, the plate holesremain fixed in relation to each other and to the longitudinal axis ofthe plate. Consequently, undesirable loads may be imposed on the platefasteners as vertebral bodies subside after a spacer and/or bone graftmaterial is implanted in the intervertebral disc space of adjacentvertebrae.

Further, screw blocking systems are generally provided in a bone plateto keep the fasteners from backing out of the plate. In the presentinvention, each opening in the plate preferably has a groove or recessfor receiving a split ring, though any other suitable screw lockingsystems may be used in connection with the present invention.

Split rings may be pre-assembled to the bone plate. A split-ring can besized to expand upon insertion of a bone screw into an opening in thebone plate. Once the head of the screw has passed through the splitring, the split ring can contract under its natural spring tension. Whenthe ring relaxes to its unexpanded state, it prevents the bone screwfrom backing out of the plate by the engagement of an undersurface ofthe split-ring and an upwardly facing surface on the bone screw. U.S.Pat. No. 6,602,255, titled “BONE SCREW RETAINING SYSTEM” and issued onAug. 5, 2003 and U.S. Pat. No. 6,261,291, titled “Orthopedic ImplantAssembly” and issued on Jul. 17, 2001, both disclose devices used forsecuring bone screws to a bone plate and are incorporated herein byreference in their entirety as if fully set forth herein.

Generally, after implanting the spacer between a pair of vertebrae,there is a compression of the spacer between the adjacent vertebralbodies. This compression ensures a good engagement between the spacerand the endplates, increasing the chances that fusion will occur. Often,particularly in the period immediately following surgery, the spacer maysubside slightly into the endplates. In the case of allograft spacers,the space between the vertebral endplates may decrease due to graftresorption.

Where a rigid fixation plate is used to connect vertebral bodies, thissubsidence may tend to shift more of the spinal load to the plate thanis desirable. Such load shifting can also occur due to inaccuracies ininstalling the plate to the vertebrae. In extreme circumstances, thisload shifting can result in non-fusion or incomplete fusion between theadjacent vertebral bodies.

It is known in the art to provide a spinal plate which may be adjustablealong the longitudinal axis between a plurality of positions. Theseplates may generally be described as incremental locking plates that arenot infinitely adjustable. Such plates only allow for a first plate anda second plate to be assembled in a finite number of fixed positionswith respect to one another by a surgeon or through natural subsidenceafter implantation. Moreover, many of the plates cannot be extended oncelocked in a fixed position, and this restricts flexibility duringsurgery and in revisions.

Accordingly, there exists a need for a fixation system having platessusceptible to infinite adjustment between a first and second assembledposition. Such a system preferably includes plates that may freelysubside in a first direction, while preventing movement of the plates ina second direction, as well as having the ability to unlock so that theplates can move in the second direction if desirable or necessary. Sucha plate preferably provides the desired support to the vertebrae to befused, and allows compression of the vertebrae with respect to at leasta portion of the plate, thereby limiting the undesirable effects of loadshielding by the plate due to graft subsidence caused by settling ornormal forces experienced in the spinal column.

SUMMARY OF THE INVENTION

A first aspect of the present invention is a dynamic subsidence device.In accordance with one embodiment of the present invention, the dynamicsubsidence device comprises a first member, a second member contactingthe first member, the first and second members being moveable withrespect to one another. Preferably, the device further includes a rampportion on the first member, an interference portion on the secondmember, and a bearing member situated between the ramp portion of thefirst member and the interference portion of the second member.Alternatively, the ramp portion may be adapted to the second member, andthe interference portion may be adapted to the first member.

Preferably, the bearing member permits movement of the first member withrespect to the second member in a first direction in an infinite numberof positions between a first assembled position and a second assembledposition. Preferably, the bearing member prevents movement of the firstmember with respect to the second member in a second direction betweenthe first and second assembled positions.

In accordance with one embodiment of this first aspect of the presentinvention, the ramp portion of the first member is preferably a curvedramp.

In accordance with another embodiment of this first aspect of thepresent invention, the first member may further include at least onevertical slot. Preferably, the second member further includes at leasttwo apertures, wherein the first and second member are prevented fromdisengaging when the vertical slot of the first member is situatedbetween the at least two apertures and a pin is placed through the atleast two apertures of the second member and the vertical slot of thefirst member. Alternatively, the second member may include at least onevertical slot and the first member may include at least two apertureswherein the first and second member are prevented from disengaging whenthe vertical slot of the second member is situated between the at leasttwo apertures and a pin is placed through the at least two apertures ofthe first member and the vertical slot of the second member.

The function of the vertical slot in the plate system is to aid inlimiting the translation of the first and second members with respect toone another between the first and second assembled position. Further,the function of at least one of the at least two apertures in the platesystem is to secure the pin member therein. Preferably, the pin isconfigured to slidably engage the slot of the first or second member.Preferably, the structure of the slot and pin configuration of the platesystem functions to add further security to the slidable engagement ofthe first and second members with respect to one another.

In accordance with yet another embodiment of this first aspect of thepresent invention, the first member includes at least one opening forreceiving a fastener therein for attaching the first member to avertebral body of the spine. Preferably, the second member furtherincludes at least one opening for receiving a fastener therein forattaching the second member to a vertebral body of the spine. Theopenings in the first and second members adapted to receive fastenerstherein preferably include a groove or recess for receiving a split ringtherein. Preferably, each split ring adapted to an opening is configuredto prevent a fastener from backing out of the first and second membersby the engagement of an undersurface of the split ring and an upwardlyfacing surface on the fastener.

In accordance with still yet another embodiment of this first aspect ofthe present invention, the first member may further include a firstprong and a second prong, the first prong having a male portion and areceiving portion, the second prong including the ramp portion. Thesecond member may further include a first prong and a second prong, thefirst prong having a female portion configured to receive the maleportion of the first prong of the first member and a guidance portionconfigured to engage the receiving portion of the first prong of thefirst member, the second prong of the second member including theinterference portion.

In accordance with still yet another embodiment of this first aspect ofthe present invention, the bearing member is freely seated between theramp portion and the interference portion of the lock assemblyassociated with the first and second plate members when the first andsecond members are in the first assembled position. Further, the bearingmember is preferably freely seated between the ramp portion and theinterference portion of the lock assembly when the first member ismoving with respect to the second member in a first direction.Alternatively, the bearing member preferably locks between the rampportion and the interference portion of the lock assembly when the firstmember is moving with respect to the second member in an opposite seconddirection thereby impeding the translation of the plates in the seconddirection. Preferably, as soon as the first and second members areimpeded by the lock assembly from translating in the second direction,the bearing member may return to the portion of the ramp portion whereit is freely seated between the ramp portion and the interferenceportion. In this position, the bearing member may rotate freely and thefirst and second members may subside again while moving in the firstdirection.

In accordance with still yet another embodiment of this first aspect ofthe present invention, the lock assembly preferably includes a keyholeconfigured to allow an instrument to enter a space between the rampportion and the interference portion and hold the bearing member inplace, thus releasing the bearing member locked between the ramp portionand the interference portion. Preferably, the instrument is configuredto impart a force on the bearing member sufficient to overcome thefrictional forces on the bearing member while locked between the rampportion and the interference portion. Preferably, the instrument isfurther configured to move the bearing member back towards the deeperpart of the ramp portion where the bearing member may freely rotate.

A second aspect of the present invention is a dynamic subsidence platefor the cervical vertebrae of the spine. In accordance with oneembodiment of this second aspect, the dynamic subsidence plate comprisesa first member having a protruding end including a ramp portion, asecond member having a receiving end configured to slidably receive theprotruding end of the first member, the receiving end of the secondmember including an interference portion, and a bearing member situatedbetween the ramp portion of the first member and the interferenceportion of the second member.

In accordance with this second aspect of the present invention, thebearing member is preferably configured to allow movement of the firstmember with respect to the second member in a first direction in aninfinite number of positions between a first assembled position and asecond assembled position. Preferably, the bearing member is furtherconfigured to prevent movement of the first member with respect to thesecond member in an opposite second direction between the first andsecond assembled positions.

A third aspect of the present invention is a dynamic subsidence platefor the cervical vertebrae of the spine. In accordance with oneembodiment of this third aspect, the dynamic subsidence plate comprisesa first member having a first prong and a second prong, the first pronghaving a male portion and a receiving portion, the second prong having aramp portion. The plate preferably further comprises a second member forreceiving the first member, the second member having a first prong and asecond prong, the first prong having a female portion configured toreceive the male portion of the first prong of the first member and aguidance portion configured to engage the receiving portion of the firstprong of the first member, the second prong of the second member havingan interference portion. Preferably, the plate further comprises abearing member freely seated between the ramp portion of the secondprong of the first member and the interference portion of the secondprong of the second member when the first member is moving with respectto the second member in a first direction and alternatively lockedbetween the ramp portion of the second prong of the first member and theinterference portion of the second prong of the second member when thefirst member is moving with respect to the second member in an oppositesecond direction.

An exemplary method of providing dynamic subsidence between a first andsecond body with the plate device of the present invention includesslidably engaging the first and second plate members together andmaintaining the first and second plate members in a first assembledposition. The method including fastening the first member to the firstbody and the second member to the second body and allowing the first andsecond plate members to subside in a first direction in an infinitenumber of positions between the first assembled position and a secondassembled position, wherein the first body and the second body arecloser together when the plate device is in the second assembledposition.

Preferably, the plate device further includes a lock assembly having aramp portion, an interference portion, and a bearing member, wherein thebearing member is located in a deeper portion of the ramp portion whenthe plate device is in the first assembled position. Preferably, thebearing is lodged between the ramp portion and the interference portionof the lock assembly when the plate is in the second assembled position,the bearing preventing the first and second plate members from subsidingfurther in the first direction.

The method may further include inserting an instrument between the rampportion and the interference portion such that the instrument may beused to release the lodged bearing.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the subject matter of the presentinvention and the various advantages thereof can be realized byreference to the following detailed description in which reference ismade to the accompanying drawings in which:

FIG. 1 is an exploded front view of a dynamic subsidence plate accordingto a first embodiment of the present invention.

FIG. 2 is an exploded rear view of the dynamic subsidence plate of FIG.1.

FIG. 3 is a top view of the second member of the dynamic subsidenceplate of FIG. 1.

FIG. 4 is a top view of the second member of the dynamic subsidenceplate of FIG. 1.

FIG. 5 is a front assembled view of an exemplary embodiment of a firstmember, a second member, and a clip, the clip used to maintain the firstmember and the second member in a first assembled position.

FIG. 6 is a back assembled view of the assembly of FIG. 5.

FIG. 7 is a front assembled view of the plate of FIG. 1 showing thevertical slot of the first member situated between the at least twoapertures of the second member and a pin placed through the at least twoapertures of the second member and the vertical slot of the firstmember.

FIG. 8 is a back assembled view of the plate of FIG. 1 showing aninstrument inserted into the keyhole of the first member.

FIG. 9 is a cross-sectional view taken along line 3-3 of the lockassembly of the dynamic subsidence plate of FIG. 1 when the first memberand the second member are in an exemplary first assembled position.

FIG. 10 is a cross-sectional view taken along line 4-4 of the lockassembly of the dynamic subsidence plate of FIG. 1 when the first memberand the second member are in a locked position.

FIG. 11 is a cross-sectional view taken along line 5-5 of the lockassembly of the dynamic subsidence plate of FIG. 1 when the first memberand the second member are in an exemplary second assembled position.

FIG. 12 is a cross-sectional view of an alternative dynamic subsidenceplate showing the ramp portion of the lock assembly when the firstmember and the second member are in an exemplary first assembledposition, the ramp portion shown as an inclined plane.

FIG. 13 is a cross-sectional view of the plate of FIG. 12 showing theramp portion of the lock assembly when the first member and the secondmember are in a locked position, the ramp portion shown as an inclinedplane.

FIG. 14 is a cross-sectional view of the plate of FIG. 12 showing thelock assembly when the first member and the second member are in anexemplary second assembled position, the ramp portion shown as aninclined plane.

DETAILED DESCRIPTION

As used herein, when referring to bones or other parts of the body, theterm “proximal” means closer to the heart and the term “distal” meansmore distant from the heart. The term “inferior” means lower or bottomand the term “superior” means upper or top. The term “anterior” meanstowards the front part of the body or the face and the term “posterior”means towards the back of the body. The term “medial” means toward themidline of the body and the term “lateral” means away from the midlineof the body.

Referring to the drawings, wherein like reference numerals refer to likeelements, there is shown in FIGS. 1-11, an embodiment of the dynamiccervical plate of the present invention designated generally byreference numeral 10. As shown in those figures, plate 10 includes afirst plate member 12, a second plate member 14, and a lock assembly 16.Preferably, first member 12 and second member 14 are slidably engagable.

Preferably, lock assembly 16 includes a ramp portion 18 on first member12, an interference portion 20 on second member 14, and a bearing member19 situated between ramp portion 18 of first member 12 and interferenceportion 20 of second member 14. Alternatively, ramp portion 18 may beadapted to second member 14 and interference portion 20 may be adaptedto first member 12. Preferably, ramp portion 18 is either an inclinedplane or a curved ramp, while it is contemplated ramp portion 18 may beother geometric or non-geometric configurations.

Preferably, lock assembly 16 is configured to allow movement of firstmember 12 with respect to second member 14 in a first direction in aninfinite number of positions between a first assembled position and asecond assembled position. Preferably, lock assembly 16 substantiallyprevents movement of first member 12 with respect to second member 14 inan opposite second direction between the first and second assembledpositions.

Preferably, first member 12 includes a mating surface 22 configured tosubstantially engage a mating surface 24 of second member 14. Thesliding engagement of first member 12 and second member 14 in the firstdirection can no longer occur after surfaces 22, 24 mate.

Preferably, when plate 10 is in the first assembled position, firstmember 12 and second member 14 are in sliding engagement. Further,bearing member 19 is preferably freely seated between ramp portion 18and interference portion 20 in the first assembled position. Preferably,a surgeon preoperatively decides the amount of subsidence between firstmember 12 and second member 14 of plate 10 that is needed for aparticular patient.

Depending on the particular patient, plate 10 is generally configured tosubside 1 mm to 10 mm. Generally, surface 22 of first member 12 andsurface 24 of second member 14 are slidably engaged and distancedapproximately 1 mm to 4 mm apart in the first assembled position. Asurgeon may decide based on the type of operation performed or theparticular patient's anatomy and/or deformity to have surface 22 offirst member 12 and surface 24 of second member 14 in the firstassembled position at a greater distance than 4 mm. The second assembledposition is defined as the position where sliding engagement of firstmember 12 and second member 14 in the first direction is prevented,generally, after surfaces 24, 26 mate.

In a preferred embodiment, first member 12 may translate freely withrespect to second member 14 in a first direction D1 without engaginglock assembly 16. Preferably, lock assembly 16 is configured to allowfirst member 12 and second member 14 to slide freely with respect to oneanother in first direction D1. Generally, first direction D1 is adownward direction as surface 22 of first member 12 translates towardsecond member 14. Movement in the first direction may only occur forfirst member 12 if second member 14 is in a fixed position. However,because of the anatomical structure and function of the spine, first andsecond members 12, 14 will generally both translate with respect to oneanother as subsidence occurs. Therefore, first member 12 and secondmember 14 both generally define movement in first direction D1 andsecond direction D2. It is also contemplated that the present inventionis applicable to implants other than plates and further, other thanplates moving in directly opposite directions.

As shown in FIGS. 9-11, lock assembly 16 preferably provides a lockingmechanism to allow movement or translation of members 12, 14 freely infirst direction D1, while preventing backward movement or translation ofmembers 12, 14 in reference to one another in second direction D2.Preferably, bearing member 19 is located in the deeper part of rampportion 18 when first member 12 and second member 14 are in slidingengagement at an exemplary first assembled or at rest position as shownin FIG. 9.

As first member 12 and second member 14 begin to subside, bearing member19 is preferably static and will rotate in place in the deeper area ofthe pocket. As shown in FIG. 10, if first member 12 and second member 14begin to move or translate in opposite second direction D2 and members12, 14 therefore start to pull apart, bearing member 19 will be forcedto rotate up ramp portion 18 due to the friction created between rampportion 18 and interference portion 20 of members 12, 14.

Preferably, bearing member 19 will rotate up ramp portion 18 until theamount of point loading of bearing member 19 with respect to rampportion 18 and interference portion 20 of members 12, 14 is sufficientenough to stop the movement or translation of members 12, 14 in seconddirection D2. Preferably, this point loading occurs instantaneously asthe movement of members 12, 14 change from first direction D1 to seconddirection D2.

Preferably, movement of members 12, 14 in second direction D2 is lessthan 1 mm Preferably, lock assembly 16 is further configured such thatif first member 12 and second member 14 have stopped moving apart indirection D2, and start moving towards each other again in direction D1,bearing member 19 may go back down ramp portion 18, thus releasing lockassembly 16.

The depth and shape of ramp portion 18, interference portion 20, and thesize of bearing 22 are all critical features for performance and designintent of dynamic plate 10. If the dimensions of ramp portion 18 andinterference portion 20 are constant among several different plates, thedistance between the first and second assembled positions of plate 10may be affected by the size of bearing member 19.

Further, the locking forces imparted by ramp portion 18 and interferenceportion 20 on bearing member 19 may be affected by the size of bearingmember 19. One skilled in the art would easily understand that a largerbearing between similarly sized ramp portions 18 and interferenceportions 20 of different plates 10 would place a greater force onbearing member 19 as well as limit the distance that the plates maytranslate in the first and/or second directions between the first andsecond assembled positions.

FIGS. 12-14 show an alternative embodiment of a lock assembly 16′ for adynamic plate 10′. Preferably, lock assembly 16′ includes an inclinedplane ramp portion 18′. Lock assembly 16′ preferably provides a lockingmechanism to allow movement or translation of a plate member 12′ and aplate member 14′ freely in first direction D1, while preventing backwardmovement or translation of members 12′, 14′ in reference to one anotherin second direction D2. Preferably, bearing member 19′ is located in thedeeper part of ramp portion 18′ when first member 12′ and second member14′ are in sliding engagement at an exemplary first assembled or at restposition as shown in FIG. 12.

As first member 12′ and second member 14′ begin to subside, bearingmember 19′ is preferably static and will rotate in place in the deeperarea of the pocket. As shown in FIG. 13, if first member 12′ and secondmember 14′ begin to move or translate in opposite second direction D2and members 12′, 14′ therefore start to pull apart, bearing member 19′will be forced to rotate up ramp portion 18′ due to the friction createdbetween ramp portion 18′ and interference portion 20′ of members 12′,14′.

Preferably, bearing member 19′ will be forced to rotate up ramp portion18′, until the amount of point loading of bearing member 19′ withrespect to ramp portion 18′ and interference portion 20′ of members 12′,14′ is sufficient enough to stop the movement or translation of members12′, 14′ in second direction D2. Preferably, this point loading occursinstantaneously as the movement of members 12′, 14′ change from firstdirection D1 to second direction D2.

Preferably, movement of members 12′, 14′ in second direction D2 is lessthan 1 mm. Preferably, lock assembly 16′ is further configured such thatif first member 12′ and second member 14′ have stopped moving apart indirection D2, and start moving towards each other again in direction D1,bearing member 19′ may go back down ramp portion 18′, thus releasinglock assembly 16′.

In FIG. 2, an exploded rear view of first member 12 and second member 14of plate 10 is shown. Interference portion 20 is shown, while rampportion 18 is now hidden on the other side of first member 18.Preferably, a slight protrusion 21 extends outwardly from interferenceportion 20. Preferably, first member 12 further includes a recessedportion 23 as shown in FIG. 1. Protrusion 21 is preferably configured toengage recessed portion 23. The engagement of protrusion 21 and recessedportion 23 acts to further secure first member 12 and second member 14in an exemplary second assembled position as shown in FIG. 11. Further,protrusion 21 and recessed portion 23 may act to guide the slidingengagement of members 12, 14 between the first and second assembledpositions.

In one embodiment, first member 12 further includes at least onevertical slot 26. Preferably, second member 14 further includes at leasttwo apertures 28, wherein first and second member 12, 14 are preventedfrom disengaging when vertical slot 26 of first member 12 is situatedbetween at least two apertures 28. Further, a pin 17 is preferablyplaced through at least two apertures 28 of second member 14 andvertical slot 26 of first member 12 as shown for example, in FIGS. 5 and6. Preferably, pin 17 is a deformable rivet that may be inserted into afirst aperture 28 as shown in FIGS. 5 and 7, and later be deformed toengage and be fixed within second aperture 28 as shown in FIG. 8.

Alternatively, second member 14 may include at least one vertical slot26 and first member 12 may include at least two apertures 28 whereinfirst and second members 12, 14 are prevented from disengaging whenvertical slot 26 of second member 14 is situated between at least twoapertures 28. In this alternative embodiment, pin 17 is placed throughat least two apertures 28 of first member 12 and vertical slot 26 ofsecond member 14.

Preferably, the function of vertical slot 26 is to aid in limiting thetranslation of first and second members 12, 14 with respect to oneanother between the first and second assembled positions. Further,apertures 28 in first and second members 12, 14 preferably act to securepin 17 therein. Preferably, pin 17 is configured to slidably engage theslot of first and second members 12, 14. Further still, the slot 26 andpin 17 configuration preferably acts to add further security to theslidable engagement of first and second members 12, 14.

Preferably, vertical slot 26 includes a top portion 25 and a bottomportion 27. When the first and second members 12, 14 are in the firstassembled position, pin 17 is preferably through slot 26 and apertures28 and is adjacent to bottom portion 27 of slot 26. Alternatively, whenthe first and second members 12, 14 are in the second assembledposition, pin 17 is preferable through slot 26 and apertures 28 and isadjacent to top portion 25 of slot 26.

Openings 38 in plate 10 for receiving bone screws may be seen in plate10. Typically, spinal plates are secured to adjacent vertebrae by bonescrews which pass through openings in the plates. Screw blocking systemsare provided to keep the vertebral screws from backing out of the plate.In the present invention, each opening 38 in members 12, 14 preferablyhas grooves or recesses for receiving a split ring, though any othersuitable screw locking systems may be used.

Plate 10 may further include a keyhole 40 associated with either firstand/or second members 12, 14. Preferably, keyhole 40 is configured toallow an instrument 42 to enter at least some of the space between rampportion 18 and interference portion 20 as shown generally in FIG. 8.Preferably, instrument 42 is configured to dislodge bearing member 19from between ramp portion 18 and interference portion 20 and/or holdbearing member 19 in place in the deeper portion of ramp portion 18.Instrument 42 may be used to release bearing member 19 locked betweenramp portion 18 and interference portion 20.

Preferably, instrument 42 is configured to impart a force on bearingmember 19 sufficient to overcome the frictional forces on bearing member19 while locked between ramp portion 18 and interference portion 20.Preferably, instrument 42 is further configured to move bearing member19 back towards the deeper part of ramp portion 18 where bearing member19 may freely rotate as shown in FIG. 9.

FIGS. 3 and 4 are top views of members 12 and 14 respectively. As shownin FIG. 3, first member 12 preferably includes a first prong 29 and asecond prong 31 extending outwardly from surface 22. Preferably, firstprong 29 includes a male portion 30 and a receiving portion 32 andsecond prong 31 includes ramp portion 18. As shown in FIG. 4, secondmember 14 preferably includes a first prong 33 and a second prong 35.Preferably, first prong 33 includes a female portion 34 and a guidanceportion 36. Female portion 34 is preferably configured to receive maleportion 30 of first prong 29 of first member 12 and guidance portion 36is preferably configured to engage receiving portion 32 of first prong29 of first member 12. Preferably, second prong 35 of second member 14includes interference portion 20.

FIG. 5 is a view of first and second members 12, 14 of plate 10 in anexemplary first assembled position. Preferably, a clip 50 is used tomaintain the spacing between surfaces 22, 24 of first and second members12, 14 in the first assembled position. Clip 50 preferably includes atop portion 51, a first end 52, a bottom portion 53, and a second end54. Preferably, in the first assembled position, surface 22 of firstmember 12 rests on top portion 51 of clip 50 and bottom portion 53 ofclip 50 rests on surface 24 of second member 14.

As shown in FIG. 6, clip 50 may encompass a portion of the perimeter ofmembers 12, 14 such that a surgeon or medical technician has access toends 52, 54 after members 12, 14 have been implanted. It is preferred,that clip 50 functions to maintain the spacing of members 12, 14 in thefirst assembled position while also being configured to easily beremoved from between members 12, 14. Alternatively, clip 50 may encirclethe entire perimeter of members 12, 14. Preferably, clip 50 is made of amaterial strong enough to withstand forces that may be generated betweenmembers 12, 14 before and after implantation of members 12, 14 to arespective vertebral body.

One method of implanting plate 10 includes fastening first and secondmembers 12, 14 in the first assembled position to a respective vertebralbody, clip 50 used to maintain members 12, 14 of plate 10 in the firstassembled position, and removing clip 50 from the assembly such thatfirst member 12 and second member 14 may translate in a first directionD1. In this exemplary method, plate 10 further includes lock assembly 16configured to allow first and second members 12, 14 to subside in afirst direction in an infinite number of positions between a firstassembled position and a second assembled position and alternativelylimit the translation of the first member 12 and the second member 14 inan opposite second direction D2.

Behind assembled clip 50 in the first assembled position, bearing member19 is generally located in the deeper part of ramp portion 18. Afterclip 50 is removed from between members 12, 14, bearing member 19 ispreferably static and will rotate in place in the deeper area of thepocket as surfaces 22, 24 of members 12, 14 slowly come together asnatural subsidence occurs. If during subsidence, first member 12 andsecond member 14 begin to move or translate in second direction D2,bearing member 19 will be forced to rotate up ramp portion 18 due to thefriction created between ramp portion 18 and interference portion 20 ofmembers 12, 14.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A dynamic subsidence device comprising: a first plate member having first and second prongs defining a first portion of a central elongated aperture; a second plate member having first and second prongs defining a second portion of the central elongated aperture, the first and second plate members being translatable with respect to one another when the first and second prongs of the second plate member are operatively coupled to the first and second prongs of the first plate member; and a pin member having a head and a shaft, the shaft of the pin member being located within at least a portion of each of the first and second plate members when the first and second plate member are operatively coupled to one another, wherein a length of the central elongated aperture varies as the first and second plate members translate with respect to one another.
 2. The dynamic subsidence device of claim 1, wherein the first plate member further includes at least one lateral elongated slot.
 3. The dynamic subsidence device of claim 2, wherein the second plate member further includes at least two lateral apertures in one of the first or second prongs thereof.
 4. The dynamic subsidence device of claim 3, wherein the first and second plate members are prevented from disengaging when the lateral elongated slot is situated between the at least two lateral apertures and the pin is placed through the at least two lateral apertures and the lateral elongated slot.
 5. The dynamic subsidence device of claim 1, wherein the first plate member includes at least one opening for receiving a fastener therein for attaching the first plate member to a first vertebral body of the spine, and wherein the second plate member includes at least one opening for receiving a fastener therein for attaching the second plate member to a second vertebral body of the spine.
 6. The dynamic subsidence device of claim 1, wherein the first and second prongs of the first plate member each having a male portion and a receiving portion.
 7. The dynamic subsidence device of claim 6, wherein the first and second prongs of the second plate member each having a male portion and a receiving portion that correspond to the male and receiving portion of the first and second prongs of the first plate first member.
 8. The dynamic subsidence device of claim 7, wherein a u-shaped wall defines the receiving portion of each of the first and second prongs of the first plate member.
 9. The dynamic subsidence device of claim 1, wherein the central elongated aperture has proximal and distal ends, the proximal and distal ends each having a diameter greater than a width defined by a central portion of the central elongated aperture separated by the first and second prongs of each of the first and second plate members.
 10. A dynamic subsidence device comprising: a first plate member having a body portion and first and second prongs extending inferiorly from the body portion, the first and second prongs defining a first portion of a central elongated aperture; a second plate member having a body portion and first and second prongs extending superiorly from the body portion, the first and second prongs defining a second portion of the central elongated aperture, the first and second plate members being translatable with respect to one another when the first and second prongs of the second plate member are operatively coupled to the first and second prongs of the first plate member; and a pin member having a head and a shaft, the shaft of the pin member being located within at least a portion of each of the first and second plate members when the first and second plate member are operatively coupled to one another, wherein a length of the central elongated aperture varies as the first and second plate members translate with respect to one another.
 11. The dynamic subsidence device of claim 10, wherein the first plate member further includes at least one lateral elongated slot.
 12. The dynamic subsidence device of claim 11, wherein the second plate member further includes at least two lateral apertures in one of the first or second prongs thereof.
 13. The dynamic subsidence device of claim 12, wherein the first and second plate members are prevented from disengaging when the lateral elongated slot is situated between the at least two lateral apertures and the pin is placed through the at least two lateral apertures and the lateral elongated slot.
 14. The dynamic subsidence device of claim 10, wherein the first plate member includes at least one opening for receiving a fastener therein for attaching the first plate member to a first vertebral body of the spine, and wherein the second plate member includes at least one opening for receiving a fastener therein for attaching the second plate member to a second vertebral body of the spine.
 15. The dynamic subsidence device of claim 10, wherein the first and second prongs of the first plate member each having a male portion and a receiving portion.
 16. The dynamic subsidence device of claim 15, wherein the first and second prongs of the second plate member each having a male portion and a receiving portion that correspond to the male and receiving portion of the first and second prongs of the first plate firstmember.
 17. The dynamic subsidence device of claim 16, wherein a u-shaped wall defines the receiving portion of each of the first and second prongs of the first plate member.
 18. The dynamic subsidence device of claim 10, wherein the central elongated aperture has proximal and distal ends, the proximal and distal ends each having a diameter greater than a width defined by a central portion of the central elongated aperture separated by the first and second prongs of each of the first and second plate members.
 19. A dynamic subsidence device comprising: a first plate member having a body portion and first and second prongs extending inferiorly from the body portion, the body portion and the first and second prongs defining a first portion of a central elongated aperture; a second plate member having a body portion and first and second prongs extending superiorly from the body portion, the body portion and the first and second prongs defining a second portion of the central elongated aperture, the first and second plate members being translatable with respect to one another when the first and second prongs of the second plate member are operatively coupled to the first and second prongs of the first plate member; and a pin member having a head and a shaft, the shaft of the pin member being located within at least a portion of each of the first and second plate members when the first and second plate member are operatively coupled to one another, wherein a length of the central elongated aperture varies as the first and second plate members translate with respect to one another.
 20. The dynamic subsidence device of claim 19, wherein the central elongated aperture has proximal and distal ends, the proximal end located in the body portion of the first plate member and the distal end located in the body portion of the second plate member, the proximal and distal ends each having a diameter greater than a width defined by a central portion of the central elongated aperture separated by the first and second prongs of each of the first and second plate members. 